Abstract To quantify the performance drop of the air source heat pump (ASHP) system under a special kind of mal-defrost phenomenon appearing in moderate climate conditions, a field test was conducted for 8days at the initial stage of a heating season in Beijing, China. The mal-defrost was found with the more than 60% frosted area of the outdoor heat exchanger after the system running 5days. During this frosting period, the system COP was significantly degraded, only 2.3 under an environment temperature of 7.9°C. Comparing the test data before and after frosting, it was found that the mal-defrost decreased the COP up to 40.4% and the heating capacity to 43.4%. Such low energy efficiency continued quite a long time until the defrost control was started up manually by the authors. After defrosting, the COP reclaimed to the normal level of 5.0. The origins of this special mal-defrost phenomenon were discussed. And some suggestions were proposed to modify the current defrosting control strategy, which were helpful to avoid the mal-defrost problem in the current ASHP system and therefore improve the system performances.